1. Field of Invention
The present invention relates to display devices and, in particular, to driver arrangements for display devices.
2. Description of Related Art
A variety of display devices, such as liquid crystal displays or light emitting diode (LED) displays, are in widespread use. Recently, a further type of LED display has been proposed in the form of an addressable electroluminescent display. The electroluminescent display device comprises a mix of organic materials such as organic polymers or small molecules sandwiched between an anode and a cathode supported by a solid substrate, such as, for example, a glass, plastics or silicon substrate, the organic materials providing the light emitting elements of the display.
Organic material LED's have a much sharper response characteristic than liquid crystal display devices. The organic LED devices have very sharp ‘turn on’ and ‘turn-off’ characteristics in response to an applied current which provides such displays with improved contrast in comparison to liquid crystal displays. In addition to improved contrast, the organic materials are also considered to provide significant benefit in terms of fabrication.
For organic LED displays incorporating organic polymer materials as the light emitting pixels, the organic polymer materials may be deposited on the substrate using fabrication techniques which cannot be adopted to manufacture liquid crystal or conventional light emitting diode displays. One method which has been proposed is to deposit the organic polymer materials onto the substrate using inkjet printing in which the polymer materials are deposited as discrete drops of the material onto predisposed deposition sites provided as a matrix configuration on the substrate. The use of inkjet printing can be particularly advantageous for colour displays because the various organic polymer materials which comprise the red, green and blue LED's at each pixel of the display can be deposited in the required predefined patterns without the need for any etch process steps. In the case of small molecule type organic LED displays, shadow mask evaporation is generally applied to form the colour pixels.
Furthermore, as the active materials of the display are organic polymer materials, they may be deposited onto any suitable substrate material, including plastics materials in the form of a continuous, flexible and spoolable sheet. The characteristics of the organic polymer materials lend themselves, therefore, to the fabrication of very large area monochrome or colour display devices containing very large numbers of rows and columns of the pixels of active material making up the display area of the display device.
An organic electroluminescent display may be driven using either an active or a passive matrix addressing system. The display elements which create the light output at any pixel of the display are, in essence, provided by organic light emitting diodes. These are current driven devices, so when an active matrix addressing scheme is used to address the display to create a displayed image, two transistors per pixel of the displayed image are provided, the first to act as a switch so as to allow a data signal to be passed to a second transistor which acts as a current driver for the LED of the pixel, thereby to determine the brightness for the pixel.
A passive matrix addressing scheme is shown schematically in FIG. 1. The display element 2 shown in FIG. 1 comprises a substrate 4 supporting an array of strip-shape electrodes 6 which constitute the anode electrodes of the display element. A layer 8 of an organic photoemissive material is provided over the anode electrodes 6 and a second array of strip-shape electrodes 10, which constitute the cathode elements for the display element, are provided over the photoemissive layer 8. It can be seen from FIG. 1 that the respective arrays of anode and cathode strip-shape electrodes 6, 10 are arranged substantially orthogonal to each other. If a voltage is applied between any two of the strip-shape electrodes, a current will pass through that part of the photoemissive layer 8 arranged in the area where the two electrodes overlap. The material of the photoemissive layer behaves as a light emitting diode, and hence that part of the photoemissive layer in the overlap area of the two electrodes to which the voltage is applied will emit light. This can be seen more clearly with reference to FIG. 2.
From FIG. 2 it can be seen that the pixels of the display are each made up of an organic LED coupled between respective strip-shape anode and cathode electrodes. The strip-shape anode electrodes are, for example, decoupled from ground potential by a high impedance circuit, indicated by a value Z in FIG. 2. Data signals, indicated by voltages V1 to V4 in FIG. 2, are applied to the cathode electrodes of the array. At the same time, the strip-shape anode electrodes are selectively coupled directly to ground potential. Hence, for the example shown in FIG. 2, when the voltage V1 is applied to the left most strip-shape cathode electrode, the organic LED L1 will emit light. Likewise, when voltages V2 to V4 are applied to the cathode electrodes 10, the LED's L2 to L4 will respectively emit light.
Addressing schemes of the above type are called passive matrix schemes because there are no active elements located within the display area to drive the LED's to emit light. The light emission results purely from the data signals, in the form of voltage pulses, provided from the frame or boundary area of the display device to one of the sets of strip-shape electrodes, either the cathode or anode electrodes. However, the thin strip-shape electrodes have electrical resistance, and this electrical resistance becomes larger with increase in the length of the strip-shape electrodes. Hence, if the size of the display area of the display device is made larger, the length of the strip-shape electrodes increases and, it follows, that the electrical resistance of the strip-shape electrodes is also increased.
The displays are driven from the side edge of the display and hence, when a voltage pulse is applied to any particular electrode, the voltage actually applied to the pixels underlying that electrode will decrease with the distance of any pixel from the edge of the display to which the voltage pulse is applied due to the electrical resistance of the electrode. The potential drops along the electrodes can become significant in comparison to the LED drive voltages. It will be appreciated, therefore, that if the electrodes are relatively long, the voltage applied to a pixel located at the distal end of the electrode relative to the driven edge, will be significantly less than the voltage applied to a pixel located close to the driven edge. The brightness of the display decreases therefore with increase in distance from the driven edge and since the brightness-voltage characteristic of the LED devices is non-linear, this gives rise to non-uniform brightness of displayed image.
Additionally, the intensity of the light emitted from an LED display is a function of the peak illumination intensity of the individual LED devices and the number of lines of pixels in the actual display area of the display. This is because the LED's of the display are addressed by pulse operation in a frame period. The time period during which any LED may be addressed is known as the duty ratio and is equal to tf/N, where tf is the frame period and N is the number of lines in the display. It follows, therefore, that if the number of lines in the display is increased, the duration for which any pixel may be addressed is decreased. The peak intensity of luminance from an LED occurs when it is addressed and this is averaged over the frame period. Therefore, to provide a flicker-free display, as the size of the display area is increased, and the number of lines in the display also increases to maintain resolution, the peak intensity of light emitted from the LED devices must be compensated to maintain a required output intensity for the display because it is only possible to address the LED devices for a shorter duration during a frame cycle. This can be particularly problematical for organic LED devices because of their very fast rise and decay times which means that they do not manifest an intrinsic memory characteristic.
The peak intensity of the LED devices can be increased by increasing the voltage of the pulses used to address the LED devices. It can be appreciated, therefore, that as display size, and hence the number of scan rows, is increased, relatively high voltage pulses are required to drive the LED devices at a high current density and thereby provide sufficient light output intensity from the display. This is a considerable disadvantage, as the long term reliability of the light emitting devices can be impaired and when the display is incorporated in a device powered from an internal battery supply, such as a laptop computer, larger, heavier and more expensive batteries must be used. However, the use of such relatively high voltage pulses gives rise to further problems concerning operation of the LED's.
It is known that in LED devices, the possibility of recombination of electron hole pairs, which produces the light emission, can decrease with an increase in voltage. This is because the optimum region for operation of a LED device is what is commonly known as the ‘recombination zone’.
The operational characteristic of a typical LED is shown in FIG. 3, which shows how luminance and device efficiency vary in relation to the current and voltage applied to the device. It can be seen from FIG. 3 that, once a current threshold is reached, as the current passing through the device is increased, the luminance of the device also increases. However, with regard to efficiency, it can be seen that device efficiency peaks very quickly once the device starts to emit light. With further increase in voltage applied to the device, the efficiency falls quickly to a relatively low efficiency level, as shown in FIG. 3. For organic polymer LED's the peak efficiency occurs typically in the range of about 2.2V to about 5V, whereas, when the applied voltage is in the range of about 10V to 20V, the efficiency of the device has fallen back to such a low level that it becomes inefficient and impractical to use such LED's. Device efficiency is a key issue for many practical applications of LED displays as the equipment incorporating the display is frequently required to operate from an internal battery source.
This sharp decrease in device efficiency arises because, as the voltage applied between the anode and cathode for a LED is increased, the recombination zone migrates towards one of the device electrodes. Because, the shape of the recombination zone depends on the applied voltage, with passive matrix addressed displays it becomes increasingly difficult to provide sufficient display light output intensity because relatively high voltage pulses are required to drive the LED devices, which, in turn, means that the LED devices can no longer be operated in the optimum recombination zone and, therefore, at an acceptable level of efficiency.
To summarise therefore, display devices typically contain more than 200 lines in order to provide sufficient resolution in the displayed image. Therefore, the LED's have a relatively low duty ratio which is compensated by increasing the voltage applied to the LED's. However, this gives rise to lower operating efficiency of the LED's, which in turn decreases the luminance of the LED's, as shown by FIG. 5. These two operational difficulties are inter-related and compound each other and, furthermore, they increase disproportionately with an increase in the number of lines in the display.
Active matrix addressing schemes are, therefore, frequently adopted for LED displays. An exemplary active matrix addressing scheme for an organic polymer LED display device is shown in FIG. 4, which illustrates four pixels of the display device. An active matrix driving scheme includes arrays of row and column address lines shown as X1 and X2, Y1 and Y2, in FIG. 4. These address lines are in the form of thin conductive strips along which pixel selection signals and data signals can be fed to the pixels of the display device. Each pixel of the display device is provided with two transistors, shown as T1 and T2 in FIG. 4. Further lines are also provided along which a supply voltage Vss can be fed to the transistors at each pixel.
When it is desired to energise any particular pixel and so cause the LED located at that pixel to emit light, a select voltage pulse is supplied along a row address line, for example, row address line X1 in FIG. 4. This voltage pulse is received by the gate electrode G of transistor T1 causing transistor T1 to switch ON for the duration of the voltage pulse. Assuming that the top left pixel is required to emit light, a data signal is applied to the source of transistor T1 which is ON. The data signal, shown as Data 1, is passed by transistor T1 to a capacitor coupled to the gate electrode of transistor T2. The data signal is therefore stored as a voltage in the capacitor.
Transistor T1 operates as a switch, whereas transistor T2 operates as a current driver for the organic LED, which is coupled to the supply Vss via the transistor T2. When operating as a current driver, the current at the drain of transistor T2 will be a function of the magnitude of the voltage stored in the capacitor, which is proportional to the data signal, Data 1. Hence, the current flowing through the organic LED, which determines the illumination intensity of the LED, can be controlled by variation of the signal Data 1.
The data signals are arranged so that the LED's always emit light during operation of the display and, therefore, lower operating voltages can be used. Hence, the use of the driver transistors at each pixel of the display enables the LED's to be operated at lower operating voltages, and hence, much higher efficiency. FIG. 5 shows the typical operating efficiencies of LED displays when operated by active and passive matrix addressing schemes. The operating efficiency of the LED's is of paramount importance and is the primary reason why active matrix schemes are frequently adopted for LED displays. Because the driver transistors are located at each pixel of the display, they need to be fabricated over a relatively large area and, hence, thin film transistors (TFTs) are used as the driver transistors in an active matrix addressing scheme. Hence, active matrix displays are commonly referred to as TFT displays.
The two most common types of TFTs are those where the layer of semiconductor material comprises either polysilicon or amorphous silicon. More recently, TFTs have also been fabricated using organic molecules or polymers as the semiconductor layer. However, because of their higher carrier mobility, polysilicon TFTs are usually used as the driver transistors in active matrix displays for organic light-emitting diode displays. With organic active matrix displays, as two driver transistors need to be provided for each LED pixel of the display, the transistor fabrication costs are relatively high because of the complexity of the fabrication techniques which must necessarily be adopted. In particular, when polysilicon driver transistors are used, a high temperature process must be used to provide the polysilicon semiconductor layer. These increased costs, particularly when the display area is made larger, negate the cost advantages provided by the organic polymer materials. Non-uniformity of transistor performance is also an issue. Again, this is particularly problematical for large area displays, because the large number of transistor drivers must be fabricated over the larger area of the display, giving rise to increased processing concerns and a reduction in the yield of fully functional transistor devices. For this reason ‘redundant’ driver transistors are usually provided, further increasing the cost of the display.
As mentioned above, polysilicon has, to date, been the preferred material for TFT fabrication because of its relatively high mobility. Typically, polysilicon TFTs exhibit a mobility of between 100 to 500 cm2/Vs, whereas amorphous silicon TFTs exhibit a typical mobility of 0.1 to 1 cm2Vs, and organic TFTs exhibit a mobility of 0.001 to 0.1 cm2/Vs. Organic LEDs are current driven devices, so in the driver circuit shown in FIG. 4, it is important to maximise the drain current provided by the transistor T2.                The drain current Id of a TFT can be expressed as:        
  Id  ⁢          ⁢  α  ⁢          ⁢            μ      ⁢                          ⁢      WC        L                  Where                    μ is the mobility of the semiconductor            W is the width of the transistor channel region            L is the length of the transistor channel region            C is the capacitance of the gate                        
Therefore, the drain current Id is proportional to the mobility of the semiconductor. Furthermore, the drain current is also proportional to the channel width but inversely proportional to the channel length. Hence, if polysilicon TFTs are used for the drive circuits, the relatively high mobility enables the footprint of the transistor structure within each pixel of the display to be minimised, which is an important consideration for polysilicon and amorphous silicon type TFTs, as both devices are opaque. Because such TFTs are fabricated using high temperature processes, they are usually formed on the rear surface of the screen (the substrate) of a display in advance of the formation of the light-emitting elements and, hence, the footprint of the TFTs will not transmit light emitted by the LEDs to a viewer of the display. The proportion of the display which is able to pass the emitted light to a viewer is known as the aperture ratio, and for relatively small size displays, such as those used in mobile telephones, an aperture ratio of about only 50% is achievable. That is, only about one-half of the available display area is able to display information to a viewer, with the remaining one-half of the display area being occupied by the opaque TFTs of the driver circuits and the conductor lines used to access the pixel located driver circuits. Even for large area displays it is difficult to achieve an aperture ratio of greater than about 70 to 80%, so the reduction of illumination efficiency arising from the use of opaque polysilicon or amorphous silicon TFTs arranged towards the front viewing side of the display is significant, irrespective of the size of the display.
It is known that organic TFTs can be fabricated from organic molecules or polymers which have a band gap providing transparency to radiation in the visible spectrum. However, such transistors have relatively low mobility and thus it has not been possible, to date, to use such organic TFTs for the active matrix drive circuit shown in FIG. 4. Displays have been demonstrated where an organic TFT has been used as the switching transistor T1 but, to date, is has not been possible to use an organic TFT for the current driver transistor T2 because the low mobility of the devices means the device footprint must be made so large to provide sufficient channel width to compensate for low mobility that the transistors T1 and T2 cannot be accommodated within the area available for each pixel of the display. Hence, the advantage of using substantially transparent TFTs for the active matrix driver circuits, which would enable aspect ratios approaching 100% to be realised, has not, thus far, been possible using the known arrangements for active matrix displays.
A further concern arises from the parasitic capacitance which exists between the driver lines to the driver transistors. In liquid crystal displays, the active liquid crystal material is located between the anode and cathode driver lines. The liquid crystal layer is usually in the range of 2 to 10 microns in thickness and, therefore, the parasitic capacitance arising between the driver line and counter common electrode is relatively small. However, for organic LED displays, the organic molecular or polymer layer is very thin, typically a few hundred nanometres in thickness. Hence, the parasitic capacitance is relatively large in comparison to LCD displays and this parasitic capacitance limits the speed at which the displays may be operated, which becomes particularly problematical as the display area increases. This is because it becomes necessary to address the display at a higher speed as the size of the display becomes larger in order to maintain the quality of the displayed image, but this gives rise to conflict because of the capacitance of the electrodes. Additionally, as the display size increases, the length, and hence the electrical resistance of the driver lines also increases, which again limits the speed at which the displays may be operated.